I doubt Bloomers is running the gearbox mod tbh, it's something USA TLR drivers have played with on rear motor cars on clay.
I've only done it to be able to run a slightly lower ride height and keep driveshafts closer to level on TLR 22SCT where longer arm geometry changes plunge more.
If you are relating to this part:
http://www.exotekracing.com/22-22sct...ranny-spacers/
then you would only use the smaller shim on a mid motor modification, the larger one will not fit... remember on a rear motor car the gearbox is not connected to anti squat block/wishbone hanger and on mid car the gearbox is not connected to rear hanger/toe block
Easiest way to do it is use some shims, aluminium will give a more consistant height as plastic washers can squash under tightening.